In accordance with recent increase in resolution of camera function installed in digital cameras, smart phones, mobile PCs and the like, strengthening of contamination control of epitaxial silicon wafers to be used for image pickup devices such as CCD and CIS has come to be demanded.
Especially, pursuit of the cause for an occurrence of white defects, which are peculiar to image pickup devices, and process management therefor has been strongly requested. At present, it is speculated that the cause of the white defects is metal contamination. Especially, primary cause for the white defects is attributed to slowly diffusing metals such as Mo, W, Ti, Nb and Ta.
Examples of the source of the metal contamination in epitaxial silicon wafers include material gas used for growing epitaxial layer and cleaning gas for cleaning an interior of a chamber. Material of the chamber and a metal parts typically used for a pipe system are also the source of the metal contamination. Further, the source of the metal contamination also includes metal dusts issued from a parts (e.g. rotation unit) of an epitaxial growth apparatus.
In order to reduce contaminants caused from the epitaxial growth apparatus, the following evaluation methods have been proposed.
In a proposed method, steps including cleaning of a vapor deposition apparatus, loading of a silicon wafer, growth of an epitaxial layer and unloading of the silicon wafer are defined as one cycle, which is repeated for predetermined times to prepare a monitor wafer on which the epitaxial layer is grown for the predetermined times. Subsequently, the lifetime of the monitor wafer is measured to evaluate the cleanliness of the vapor deposition apparatus based on the lifetime value (see Patent Literature 1: JP-A-2013-162026).
In another example, a silicon wafer is subjected to a heat treatment after a target gas is introduced into a chamber in which the silicon wafer is disposed to expose the silicon wafer to the target gas, thereby diffusing metals having been contained in the target gas. Then, the lifetime of the monitor wafer after the heat treatment is measured to evaluate the metal contamination of the target gas based on the lifetime value (see Patent Literature 2: JP-A-2013-162092).
In still another example, after repeating for predetermined times a process for cleaning an interior of a chamber of a vapor deposition apparatus with vapor etching using HCl gas and heat treatment of a silicon wafer in a hydrogen atmosphere, contaminants on the surface of the silicon wafer is collected and the concentration of metal impurities contained in the contaminants is measured using ICP-MS (see Patent Literature 3: JP-A-2014-082324).
However, since the cycle is repeated for a plurality of times to provide the epitaxial layer in the above Patent Literature 1, detection sensitivity for the contaminant metal is inferior.
In Patent Literature 2, since the heat treatment is applied after exposing the silicon wafer to the target gas, the influence of the heat treatment cannot be separately evaluated.
Furthermore, since the cleaning step is included in the cycle in Patent Literatures 1 and 3, considerably long time is required for the entire treatment. In addition, since the cleaning step and the heat-treatment step are repeatedly conducted accompanying loading of the wafer into the chamber and unloading of the wafer out of the chamber, metal contaminant tends to be contained from a transfer system.